All matter is made up of elements which are fundamental substances
which cannot be broken down by chemical means. There are 92
elements that occur naturally. The elements hydrogen, carbon,
nitrogen and oxygen are the elements that make up most living
organisms. Some other elements found in living organisms are:
magnesium, calcium, phosphorus, sodium, potassium.

By the late 1800's many elements had already been discovered.
The scientist Dmitri Mendeleev, a Russian chemist, proposed
an arrangement of know elements based on their atomic mass.
The modern arrangement of the elements is known as the Periodic
Table of Elements and is arranged according to the atomic number
of elements.

Every atom would like to have an electron configuration like
a noble gases. In noble gases the outer electron shell is complete.
This makes the element chemically inert. Helium is an example
of a noble (inert) gas. It is not present in organisms because
it is not chemically reactive.

How are electrons organized around the
nucleus?

All atoms would like to attain electron configurations like
noble gases. That is, have completed outer shells. Atoms can
form stable electron configurations like noble gases

by:

losing electrons

sharing electrons

gaining electrons.

For a stable configuration each atom must fill its outer energy
level. In the case of noble gases that means eight electrons
in the last shell (with the exception of He which has two electrons).

Atoms that have 1, 2 or 3 electrons in their outer levels will
tend to lose them in interactions with atoms that have 5, 6
or 7 electrons in their outer levels. Atoms that have 5, 6 or
7 electrons in their outer levels will tend to gain electrons
from atoms with 1, 2 or 3 electrons in their outer levels. Atoms
that have 4 electrons in the outer most energy level will tend
neither to totally lose nor totally gain electrons during interactions.

The Periodic Table of Elements will show you the electron configuration
for any element you click on.

The atomic orbitals of the hydrogen atom can be visualized
as a cloud around the nucleus. The orbital represents a probability
of finding the electron at a particular location. Darker regions
signify a greater probability. Shown below are the 1s (lowest
orbital and the 2s orbital.

1s

Atomic orbitals do not always have the shape of a sphere. Higher
orbitals have very unusual shapes.

2px

These orbitals were prepared by Dr. Yue-Ling Wong from the University
of Florida for more images